The use of rivets to fasten together two sheets of material is well-known. The sheets of material are provided with apertures or holes which are formed in the sheets when positioned for fastening. The sheets are arranged in overlapping relation so that the apertures are aligned and a rivet is inserted into the aligned holes. The rivet, which generally includes a head and a body portion, is then upset to provide a structural joint between the two sheets of material.
The manner of upsetting the rivet depends on the type of rivet used and in particular on whether the rivet is a solid or blind rivet. Solid rivets are often used in fastening operations where the opposite ends of work pieces which are joined are both accessible. In other words, both the work end and the free end of the rivets are accessible during the fastening operation. Such solid rivets are generally upset at one end; that is, a load is applied to both ends of the rivet to deform the material of the rivet tail to form a structural joint. Typically, the end load on the work side of the rivet must be equal and opposite to the end load deforming the rivet tail.
Blind rivets are generally used in fastening applications in which the far side of the work pieces is inaccessible to the operator and all operations on the rivet occur from one side only; hence, the term "blind." Such applications arise frequently in the aerospace industry, where blind rivets may be used, for example, to attach body panels to an air frame. Blind rivets typically comprise a head, a sleeve or shank extending from the head, with an axially aligned bore extending through the sleeve. A stem is positioned in the axial bore and is adapted for movement inside the bore.
The upsetting of a blind rivet is generally a more complex process than the upsetting of a solid rivet. The blind rivet is inserted into a hole with at least some degree of clearance between an outside surface of a sleeve of the rivet and an inside surface defining the hole. An end load is then applied to a head of the rivet to position the rivet in the hole and retain the rivet in a relatively stationary position. The sleeve is hollow and a stem positioned therein is drawn axially upwardly through the sleeve. The stem has a portion which has an outside diameter which is greater than the inside diameter of the sleeve to expand the sleeve radially outward at the distal or "blind" end. Expansion of the blind end forms a bulbed head at the blind end of the rivet.
During the pulling process, the rivet is retained in the hole while the bulb forms at the blind end. At a predetermined time, the time being related to the forces imposed on the rivet, the stem is broken or separated such that a portion of the stem remains in the sleeve of the rivet and a portion remains in the riveting tool. Alternately, the sleeve and stem may be designed to allow the stem to pull through the sleeve without breaking the stem. This type of rivet is a "pull through" rivet.
The prior art method of blind riveting suffers from a number of problems. A problem which may arise is insufficient engagement between the rivet and the work piece at the juncture of the rivet head and sleeve or shank. Insufficient engagement, commonly termed "poor under-head hole-fill" reduces the strength of the rivet joint and can lead to premature failure of the work piece.
Another problem is that prior art methods may require a considerable force or end load to drive the rivet securely into the work piece. The need to use considerable force may have a number of undesirable consequences. These include reduced life of the riveting apparatus, strain on the operator, and possible damage to the work piece.
A further problem with conventional methods is that they are often inadequate to draw the sheets of material entirely together. Accordingly, gaps are left between the sheets of material. The gaps interfere with the formation of a tight and secure joint. Such gaps are particularly detrimental in the aerospace industry where fuel and gas tight joints are essential. Moreover, moisture may gather in the gaps, which can lead to corrosion and failure.